In the manufacture of tissue products, such as facial tissue, bath tissue, paper towels, dinner napkins and the like, a wide variety of product properties are imparted to the final product through the use of chemical additives. One common attribute imparted to tissue sheets through the use of chemical additives is softness. There are two types of softness that are typically imparted to tissue sheets through the use of chemical additives. The two types are bulk softness and topical or surface softness.
Bulk softness may be achieved by a chemical debonding agent. Such debonding agents are typically quaternary ammonium entities containing long chain alkyl groups. The cationic quaternary ammonium entity allows for the agent to be retained on the cellulose via ionic bonding to anionic groups on the cellulose fibers. The long chain alkyl groups provide softness to the tissue sheet by disrupting fiber-to-fiber hydrogen bonds within the tissue sheet.
Such disruption of fiber-to-fiber bonds provides a two-fold purpose in increasing the softness of the tissue sheet. First, the reduction in hydrogen bonding produces a reduction in tensile strength thereby reducing the stiffness of the tissue sheet. Secondly, the debonded fibers provide a surface nap to the tissue sheet enhancing the “fuzziness” of the tissue sheet. This tissue sheet fuzziness may also be created through use of creping as well, where sufficient interfiber bonds are broken at the outer tissue surface to provide a plethora of free fiber ends on the tissue surface.
A multi-layered tissue structure may be utilized to enhance the softness of the tissue sheet. In this embodiment, a thin layer of strong softwood fibers is used in the center layer to provide the necessary tensile strength for the tissue product. The outer layers of such structures may be composed of the shorter hardwood fibers, which may or may not contain a chemical debonder.
The topical or surface softness of a tissue sheet, and ultimately the resulting tissue product, may be achieved by topically applying an emollient to the surface of the tissue sheet or tissue product. One such emollient is polysiloxane. Polysiloxane treated tissues are described in U.S. Pat. No. 4,950,545, issued on Aug. 21, 1990 to Walter et al.; U.S. Pat. No. 5,227,242, issued on Jul. 13, 1993 to Walter et al.; U.S. Pat. No. 5,558,873, issued on Sep. 24, 1996 to Funk et al.; U.S. Pat. No. 6,054,020, issued on Apr. 25, 2000 to Goulet et al.; U.S. Pat. No. 6,231,719, issued on May 15, 2001 to Garvey et al.; and, U.S. Pat. No. 6,432,270, issued on Aug. 13, 2002 to Liu et al., which are incorporated by reference to the extent that they are non-contradictory herewith. A variety of substituted and non-substituted polysiloxanes may be used.
While polysiloxanes may provide improved softness in a tissue sheet, there may be some drawbacks to their use. First, polysiloxanes may be relatively expensive. Only polysiloxane on the outermost surface of the tissue sheet may contribute to topical or surface softness of the tissue sheet. Polysiloxane present within the z-direction of the tissue sheet is believed to contribute only to bulk softness, i.e., its ability to impact softness is dependent only on its ability to reduce interfiber hydrogen bonding. Interfiber hydrogen bonding may be more efficiently controlled with traditional quaternary ammonium debonding agents. When topically applied, many polysiloxanes are effective in providing surface softness to the tissue sheet. However, such polysiloxanes may also tend to be poorly retained in the wet end of the tissue making process and hence are not suitable for use in wet end applications. Topical application typically requires significant capital expense or machine modifications to employ in existing processes not set to employ topical application of polysiloxanes. Hence, there is interest in finding an effective topical polysiloxane application to a formed tissue sheet.
Polysiloxanes are also generally hydrophobic, that is, they tend to repel water. Tissue sheets or tissue products treated with polysiloxane tend to be less absorbent than tissue sheet or tissue products not containing polysiloxanes. Hydrophilic polysiloxanes are known in the art, however, such hydrophilic polysiloxanes are typically more water soluble and hence when applied to a tissue sheet will tend to migrate more in the z-direction of the sheet than the hydrophobic polysiloxanes. Hydrophilic polysiloxanes typically are also usually sold at a cost premium to the hydrophobic polysiloxanes. Hydrophilic polysiloxanes also tend to be less effective at softening and more costly to use than hydrophobic polysiloxanes. In the wet end of the tissue making process, such hydrophilic polysiloxanes are even more poorly retained on the pulp fibers than the hydrophobic polysiloxanes due to the water solubility.
Therefore, there is a need for improving the absorbency of tissue sheets containing hydrophobic polysiloxanes. There is also a need to be able to incorporate hydrophobic polysiloxanes in the wet end of the tissue making process, avoiding the need for down stream application equipment on the tissue machine. There is also a need to minimize the z-directional penetration of a polysiloxane so as to improve softness of the tissue sheet containing lower levels of the polysiloxane. By minimizing the z-directional penetration of the polysiloxane, more polysiloxane is available on the surface of the tissue sheet, thereby providing a better topical or surface softness of the tissue sheet at lower levels of polysiloxane.
There is an interest in designing economical absorbent soft tissue products containing polysiloxane. There is also an interest in improving the topical or surface softness of tissue sheets by applying a polysiloxane to the surface of a tissue sheet in a manner that minimizes the z-directional penetration of the polysiloxane. There is also an interest in incorporating hydrophobic polysiloxanes into a tissue sheet in a manner that may avoid the need for topical treatment to a formed tissue sheet while minimizing the hydrophobicity impact on the tissue sheet.